But whether penis size truly matters to a man's sex partner is up for debate. Some research shows that women who experience vaginal orgasms do so more consistently with men of bigger girth.

Other work suggests bigger isn't always better: One study found that ideal penis size depends on a person's height, and another showed that girth matters more than length to most women.

In the new study, Debby Herbenick, a researcher at Indiana University, and her colleagues asked 1,661 men to measure their penis's length and girth for the purposes of fitting for a condom. (The men submitted their data online.) At the time of their measurement, some men were with partners or "a friend," although most employed a DIY method for attaining an erection.

The average penile girth was about 4.8 inches (12.2 cm) in circumference.

Moreover, all erections are not created equal, the study also found. Different methods of obtaining an erection may lead to different size measurements — men in the study who became aroused through oral sex reported larger penis size on average than those who attained erections through fantasizing, Herbenick said.

"We don't know if that means that when men have oral sex that it's more arousing and they get a bigger erection or means that men who have bigger penises could be getting more oral sex in the first place," Herbenick told LiveScience.

Now, she'd like to gather data on female genitalia. "It would be good for women to have good data to show, 'Hey, there's a lot of diversity for women's genitals, just like there are for men's," Herbenick said.